1. Field of the Invention
A subject-matter of the present invention is a process for the preparation of ultrafine polymetallic particles, the particles obtained and the use of the particles obtained, in particular in catalysis.
2. DESCRIPTION OF THE RELATED ART
Fine metallic particles are known in various applications, in particular in catalysis. Various processes for preparing them have been disclosed.
L. K. Kurihara, et al. (Nanostructured Materials, vol. 5, No. 6, 607-613, 1995) discloses the preparation of nanometric particles of a metal by a so-called xe2x80x9cpolyolxe2x80x9d process employed for various easily reducible metals, such as, for example, Fe, Co, Ni, Cu, Ru, Rh, Pd, Ag, Sn, Re, W or Pt, alone or as a mixture. Such a process consists in dissolving or suspending a precursor of the said metal in a polyol and in then heating at reflux in order to obtain a precipitate of metallic particles. The particles obtained can be nanometric. They are polymetallic when the reaction medium comprises salts of different metals. The use of a precursor salt of aluminum or of another metal which is difficult to reduce is not mentioned. For metals other than noble metals, the dimensions of the crystallites of the particles are greater than or equal to 8 nm, more generally between 10 and 80 nm. Only Ru and Pt make it possible to obtain lower mean crystallite dimensions, respectively of 5 and 2 nm. The production of nanometric Cu-Co particles is disclosed in L. K. Kurihara et al., J. Mat. Res., vol. 10, No. 6, June 1995. In this case, two metals are involved having a standard oxidation potential between xe2x88x921.20 V and +0.8 V, that is to say easily reducible metals. The so-called xe2x80x9cpolyolxe2x80x9d process does not make it possible, however, to prepare fine particles of a metal which is difficult to reduce, such as aluminum, for example.
Nanometric particles comprising aluminum can be prepared by reducing a salt with LiAlH4. Thus, the preparation of nanocrystalline Ti/Al and Ni/Al particles from a chloride of the corresponding metal in the presence of LiAlH4 is disclosed by J. A. Haber et al. (Advanced Materials, 1996, 8, No. 2). As the reaction mechanism of this process is as follows:
Mn++nLiAlH4xe2x86x92nLi++M+nAlH3+n/2 H2
nAlH33n/2 H2
the Al content exhibits an upper limit corresponding to the valency of the associated metal. In addition, the structure of the powders obtained is difficult to control, the kinetics of decomposition of AlH3 not being easy to predict.
In addition, the preparation of nanometric intermetallic particles using borohydrides is known. S. T. Schwab et al. (Materials Science and Engineering, A204 (1995), 197-200) describe the preparation of intermetallic particles by reduction of a mixture of salts of different metals with a borohydride (for example, TiCl4+AlCl3+lithium triethylborohydride) This process makes it possible to obtain intermetallic particles without limitation with regard to the contents of the respective metals. Nevertheless, it requires a heat treatment at 1000xc2x0 C. in order to decompose the products obtained by reaction of the salts with the borohydride and to obtain the metallic forms.
Other processes are known which consist in reducing metal salts either with a borohydride or with an alkaline hydride activated by addition of BF3. In all cases, the presence of boron in the reaction medium introduces a risk of pollution.
The preparation of fine monometallic particles by reaction of a metal salt with sodium hydride in solution in an organic solvent and in the presence of an alkoxide is also known. Such a process has been described for nickel particles and for zinc particles (Paul Caubere et al., J. Am. Chem. Soc., 1982, 104, 7130; P. Gallezot, C. Leclercq, Y. Fort and P. Caubere, Journal of Molecular Catalysis, 93 (1994) 79 83, pp. 80-83; Brunet et al., Journal of Organic Chemistry, 1980, vol. 45, pp. 1937-1945). The particles obtained have a crystallite dimension of the order of an nm and are of use in particular as catalysts for heterogeneous catalyses. When they are prepared in the presence of a phosphine or 2,2xe2x80x2-bipyridine ligand, they lose their reducing properties and behave as a coupling agent (Lourak et al., Journal of Organic Chemistry, 1989, vol. 54, pp. 4840-4844).
An analogous process has also been described for the preparation of monometallic Pd particles, which can be used as catalyst for hydrogenating acetylene (J. J. Brunet and P. Caubere, J. Org. Chem., 1984, 49, 4058-4060).
This process is particularly advantageous but cannot be effectively used to prepare particles of a metal which is difficult to reduce having a standard oxidation potential at 25xc2x0 C. of less than xe2x88x921.20 V, such as aluminum, for example. Furthermore, the reduction of an Al3+ cation in the presence of sodium hydride or lithium hydride has never been touched on in these processes.
The usefulness of ultrafine polymetallic particles as catalyst in various reactions is known, in particular those which comprise aluminum or another metal, the salts of which are difficult to reduce. In point of fact, the processes of the prior art do not make it possible easily to obtain ultrafine polymetallic particles comprising aluminum of good purity and with no limitation on the aluminum content. The aim of the present invention is to provide such a process.
The present inventors have now found that, although the reduction of an aluminum salt with lithium hydride or sodium hydride gives ultrafine particles of metallic aluminum which rapidly reoxidize in the reaction medium, it is possible to obtain stable polymetallic particles comprising aluminum and another metal, there being no restriction on the Al content, by reduction of a mixture of salts with an alkali metal or alkaline earth metal hydride.
A subject-matter of the present invention is thus a process for the preparation of ultrafine polymetallic particles, the particles obtained and their uses.
The process of the present invention is a process for the preparation of ultrafine polymetallic particles by reduction of a mixture of salts in solution in an organic solvent using an alkali metal or alkaline earth metal hydride at a temperature of less than or equal to the reflux temperature of the solvent. It is characterized in that the mixture of salts in solution comprises at least one salt of a metal having a standard oxidation potential Exc2x0Mn+/M at 25xc2x0 C. of greater than xe2x88x921.18 V.
In a preferred embodiment of the invention, the mixture of salts which is reacted with the hydride additionally comprises at least one salt of a metal having a standard oxidation potential Exc2x0Mn+/M at 25xc2x0 C. of less than xe2x88x921.20 V.